I periodically remove all charging sources and watch the bank discharge with normal use over a few days. Once the bank is depleted about 400 amp hours, I bring the solar back online and keep an eye on things. Once the acceptance rate drops to about 5 amps, the bank should be very close to full but the batterymonitor still shows a 160 amp hour deficit. I've repeated this with only a 200 amp hour discharge, and when the acceptance rate drops the monitor shows 80 amp hours from full.

I know that Coulomb counting is not absolutely accurate, but a consistant 40% error seems high. I have the Peukert compensation on the monitor disabled so one amp in should be counted the same as one amp out.

Looking at the setup parameters for this device the one that is missing is charge efficiency. It probably tries to auto detect this based on Lead acid parameters,will fail at this and is assuming 60%. As a start you can make sure that it detects battery full based on the float voltage and current. Set the voltage at your end charging volts and the current at 5% initially.

If the monitor is assuming some charging inefficiency, that would explain the consistent error as the bank is replenished. I guess I don't hold out much hope either in that case.

My (limited) understanding is that the LiFePO bank should never really be float charged and I've set up the charge controllers as such. Thus, there would be no way for the battery monitor to detect the float voltage and automatically synchronize. Also, since my primary charging is solar, as sunset approaches the current drops off so I really can't use that as a trigger to sync the monitor either.

I don't mind doing a manual sync when the bank appears full. However, I would like to have a more accurate measure when the bank isn't full.

I could have been wrong the only time it synchs may be at battery full. In this case as long as it can detect that the battery is full you should be ok. These things normally do that by detecting that the battery has reached a voltage at less than a certain current, there may be a time delay as well though that is not specified, they have called this the float voltage and float current, do not get too hung up on the word float. Set this at the voltage that you consider the battery to be full at a fairly high current and it should detect battery full. After a cycle or two it should autosynch the charge efficiency.

I have a Xantrex LinkPRO and a 880Ah lead-acid bank (brand new, replacing an 800Ah AGM bank). I have the exact same problem as you described, and have had it with both battery banks. Now that I've upgraded to solar power I find the problem much easier to reproduce:
1. The bank gets charged to 100% by solar (the acceptance rate goes below 2% of capacity for a long period of time) during strong sunlight. I check this by running the generator with a Victron battery charger and that also tapers off the amps, confirming that it, too, thinks the bank is full.
2. The next morning my bank reads about -60Ah and solar charging begins
3. The charging amperage starts to taper off and goes below the 2% capacity amperage when the Xantrex still reads about -15Ah. Then the Xantrex jumps from that reading to "FULL" and I confirm that the bank is indeed full using the generator and charger as mentioned above.

This cycle repeats daily, showing that the Xantrex is 20% off. I've used a clamp-on ammeter to measure loads and that always agrees with the Xantrex reading, and shunts read the same voltage one direction as the other so I cannot explain the problem. I opened up a ticket with Xantrex last year with no luck, and opened up a new problem ticket last week since I have a reproducible test case.

I have a Xantrex LinkPRO and a 880Ah lead-acid bank (brand new, replacing an 800Ah AGM bank). I have the exact same problem as you described, and have had it with both battery banks. Now that I've upgraded to solar power I find the problem much easier to reproduce:
1. The bank gets charged to 100% by solar (the acceptance rate goes below 2% of capacity for a long period of time) during strong sunlight. I check this by running the generator with a Victron battery charger and that also tapers off the amps, confirming that it, too, thinks the bank is full.
2. The next morning my bank reads about -60Ah and solar charging begins
3. The charging amperage starts to taper off and goes below the 2% capacity amperage when the Xantrex still reads about -15Ah. Then the Xantrex jumps from that reading to "FULL" and I confirm that the bank is indeed full using the generator and charger as mentioned above.

This cycle repeats daily, showing that the Xantrex is 20% off. I've used a clamp-on ammeter to measure loads and that always agrees with the Xantrex reading, and shunts read the same voltage one direction as the other so I cannot explain the problem. I opened up a ticket with Xantrex last year with no luck, and opened up a new problem ticket last week since I have a reproducible test case.

Just a thought here I use a venerable link 2000 and it does not autosynch its parameters unless I discharge the battery to a set discharge level. You are doing very shallow cycles for your capacity. Xantrax do state that you should do a full charge discharge cycle (whatever that means) in order to synchronize the device.

Zanshin, it sounds like we have the same issue with the Xantrex - the inability to change the charge efficiency parameter as described by matrureee. With your lead acid bank, at least the monitor recognizes when you've reached voltage and current that indicate that the bank is full so it auto syncs.

I am charging almost exclusively with solar and have the MPPT controller set to bulk charge at 14 volts. At 3.5 volts per cell, the LiFePO bank is almost fully charged when the current tapers off to 5 amps. However I don't want the monitor to auto sync just because the current has dropped. It might be that the bank is full or it just might be cloudy or approaching sunset so solar output has dropped. I'm happy to manually sync when I know that the bank is full. My issue is the ever increasing error that will be displayed as the bank supplies and accepts current between syncs.

Unless someone has a better idea, I guess I'm going shopping for a battery monitor whose charge efficiency can be set much closer to 100%. LiFePO batteries will charge with very high efficiency at a very flat voltage curve.

MaineSail, I have previously read your fine article (as well as many others). I understand that the battery monitor will get out of sync over time. My problem is that it gets completely out of sync with a single discharge/charge cycle. If I take out 200 amp hours and then recharge to full, my Coulomb counter is already off by 40%.

Great article. However this device does not have a a setup parameter for charge efficiency which I would say is at the heart of this problem. The device seems to be trying to work this out on its own, in my opinion another example of modern electronics trying to be too smart for everybody´s good .
Coming to you as the guru. Do you have any practical experience with this that can give these gentlemen an insight into how it does this.

Great article. However this device does not have a a setup parameter for charge efficiency which I would say is at the heart of this problem. The device seems to be trying to work this out on its own, in my opinion another example of modern electronics trying to be too smart for everybody´s good .
Do you have any practical experience with this that can give these gentlemen an insight into how it does this.

Unfortunately that monitor is called the Link-Lite for a reason...... The Link-Pro allows far more programming options which can work much better with an LFP bank. The Victron BMV-702 also offers more....

That Link-Lite can still be used on discharge (using -Ah's) if you manually re-sync at each full charge. If you try to read it after a recharge the CEF that it is applying will be incorrect for an LFP bank...

As I feared, MaineSail. That Victron in your article looks nice. I guess I'm going shopping.

While I've got you (MaineSail) in a thread, I would be remiss if I did not take the opportunity to thank you for your help. It is not an exaggeration to credit you as an honorary crew member for all the insights your posts and articles have provided as my wife and I prepared our boat for an extended cruise.

After plowing through the voluminous LiFePO thread, I took the plunge and purchased some Balqon cells. Your experiences paved the way for a successful and safe DIY LiFePO install. We are very happy with the results and look forward to living on the hook with a versatile power bank. I learned quite a lot designing and installing the system. To be honest, we mostly just shamelessly copied your methods.

To anyone who has never visited MaineSail's marinehowto.com, you should do so immediately. I've personally used his tips and advice to rebed an entire boat worth of leaking fittings (he sells a great butyl tape on his site), service all my tapered bronze seacocks, replace a cutlass bearing, lap fit a new prop, properly install and program a smart regulator, and much more stuff that I can't think of right now.

The previous owner of my boat provided many opportunities for learning how to undo work that could be featured in The Hall of Fail. Having a resource to show the right way to do things made some of the surprises hiding on this, my first boat, a little less daunting.

Gary, I have a similar installation to yours with the exception of the monitor. I have the BlueSky Solar Boost 3024i with the IPN Proremote to monitor the battery. I have 450 watts of solar panels hooked up in parallel for 18 volts and a 500 Ah LiFePo4 battery consisting of four pieces 3.2V500AH cells in a 1P4S configuration with PCM that allows 50A charge and 100A discharge.
Even with the efficiency set on Auto versus 99% I find that the solar panels will not charge 100% even though they indicate that, usually between eleven to two o'clock each day. I also use anywhere from 50 - 120 AH overnight. Given enough days the battery voltage will eventually drop to 13.1V and I know then it is time to start the engine or plug in the shore charger which I hardly ever do because I don't do marinas too often.
The battery is not charged because when I fire up the engine with Balmar 614 it will put large amperage in for a number of hours when the monitor states 100% -00 AH. The Balmar is set up for limited amperage and proper voltage settings. It appears to reset itself at this point but it all happens again.
I have been experimenting with the voltages trying to get it to do the proper thing and have run out of ideas as well.

Canuck, it sounds like you have a charging problem not a monitoring issue. There are far more experienced people here who might be able to help you troubleshoot. I don't know what you mean by 99% or auto. What voltage have you set the solar controller to charge at?